Microsoft has released the fourth Internet Explorer 10 Platform Preview. It …

Microsoft has released the fourth preview of Internet Explorer 10. As is the case with previous Platform Previews, the release is aimed at developers: the new features are important to those creating rich, complex Web applications, but will have less impact on Web users.

However, even Web developers might struggle to get too excited about the latest preview, because they probably won't be able to run it: it only works on the Windows 8 preview release that Microsoft shipped at its BUILD conference in September.

The new features include Cross-Origin Resource Sharing (CORS), JavaScript typed arrays and binary file manipulation, and HTML5 video subtitling. Typed arrays and support for binary files enable much better performance for JavaScript applications that handle binary data, such as images and audio.

CORS provides a safe way for JavaScript applications to use services offered by different providers. Traditionally, JavaScript has been restricted by the same-origin policy: a script can only have full access to content that is hosted at the same domain, port, and protocol. This provides security by preventing theft of, for example, cookies and page data by objects included from other sites.

CORS allows one application to expose its data to another application even when the same-origin policy would otherwise deny such access. This is useful for creating "mashup" applications that combine Web services from multiple different providers.

Microsoft has positioned its Platform Previews as a way to let developers test and provide feedback on new features so that they can inform Microsoft of bugs, and guide the development of new specifications. The first two Platform Previews for Internet Explorer 10 were made available to users of Windows 7. This preview, however, is not. If you want to use it, you'll have to use the Windows 8 Developer Preview.

The third preview was in the same position; Microsoft did not release a Windows 7 version of Platform Preview 3. Instead, the version of Internet Explorer that shipped with the Windows 8 Developer Preview was the third preview. Though Internet Explorer 10 will support Windows 7 when released, Web developers wanting to test the software now will have to use an unsupported, not-even-beta operating system to do so. And while they can do so using a virtual machine, doing so will disable most or all of the hardware acceleration features found in the browser, making it a second-rate experience.

This is a decidedly odd move. Internet Explorer 10 is going to be fundamental to Windows 8 in a way that no past version of the browser has been. HTML and JavaScript are one way for developers to create new touch-friendly Metro-style applications, and this support will be built on Internet Explorer 10.

But as important as Metro-style applications are to Microsoft, the browser will still have a substantial user base on Windows 7, and the Web developers of today are far more likely to be using Windows 7 than they are Windows 8. Regular non-Metro Web applications still matter. Effectively excluding this group from the preview—the group most likely to have valuable feedback and insight—makes one wonder what the entire purpose of the scheme is.